The PCT application WO 01/19426 “Implantable Device and Method for Long-Term Detection and Monitoring of Congestive Heart Failure” has been described a measuring device, which is mounted into the pacemaker, and observes over the complications appearing in the cardiac blood vessel system and in the blood circulation in lungs. The method is based on directing various types of current/voltage excitation signals (rectangular waveform signal, sine wave signal, pulse signal, signal with varying frequency) through the bio-object and measuring the inphase and quadrature components of the electrical response to the excitation. The device measures the variations in the impedance of the cardiac blood vessel system and of the blood circulation in lungs via measuring the current flow through the object, the voltage drop forming on it and the phase shift between the excitation and response signals.
In the inventions WO 00/57953 and WO 00/57954 “A Rate Adaptive pacemaker” the device for bio-impedance measurement is used for obtaining information for adaptive control of cardiac pacing rate taking into account the energetic balance of the myocardium.
U.S. Pat. No. 5,759,159 “Method and Apparatus for Apical Detection with Complex Impedance Measurement” (Jun. 2, 1998) describes the bio-impedance measurement device used for finding the apex of a dental channel. The apex can be found by measuring the amplitude and phase characteristics of the bio-impedance between the probe and biological tissue. The method is based on measuring the amplitude and phase relationships of an electrical impedance in response to a multi-frequency excitation using a digital fast Fourier transformation (FFT).
The above described devices are not suitable for implantation because their electronic circuitry is too complicated and energy consuming.
Nowadays low voltage and low power CMOS microelectronics technology is suitable for application in switching mode analogue and digital mixed signal circuits. The extremely low power consumption is crucially important for the implantable devices operating during several years with the same battery.
Unfortunately, application of the switching mode electronics operating with pulse signals results in misleading measurement errors and measurement uncertainties due to the higher harmonics present in the pulse signals. Theoretically, application of pure sine wave signals without any higher harmonics is presumed for determination of the complex impedance. Therefore, application of the simplest rectangular waveform pulses being the most suitable for use in CMOS electronics, introduces serious measurement errors [M. Min, and T. Parve, “Improvement of the vector analyser based on two-phase switching mode synchronous detection”, Measurement, Vol. 19 (1996), No. 2, pp. 103-111].
To overcome the problem, usually the band-pass filters are introduced in order to filter out the fundamental and to suppress the higher harmonics. This solution helps to solve the higher harmonics problem only partly, because the highly selective band-pass filters have very unstable phase characteristics. The exact tuning of such filters is also rather complicated.
U.S. Pat. No. 5,063,937, A61B 5/05, “Multiple frequency bio-impedance measurement system”, B. N. Ezenwa, W. P. Couch, Nov. 12, 1991, describes the closest reference. In this document there is described a solution for a device for noninvasive measurement of the bio-impedance of a living tissue, according to which the component of interest of the excitation response of the bio-impedance (its active or reactive part) is demodulated by a synchronous detector, the reference signal of which is a rectangular wave signal being in phase or in quadrature with the excitation signal.
The systems operation is based on the switch-mode generator generating rectangular pulses, but prior to being applied to the test objects input the excitation pulses pass the highly selective band-pass filter. The band-pass filter is tuned to the main frequency of the excitation signal, and therefore the filter suppresses the higher harmonics of the original rectangular pulses, reducing in such a way the content of higher harmonics in the signals to be detected by the synchronous detector and decreasing measurement errors, which are caused by higher harmonics.
The described above solution has the following main drawbacks.
Tuning of a highly selective band-bass filter to the fundamental frequency is a troublesome procedure with an instable result. The phase shift between input and output can be compensated using sophisticated electronic circuits, which makes the excitation generator excessively complicated and bulky.
Some problems arise also in connection with generating of the reference signals used for driving the synchronous detector. In practice the rectangular reference pulse signals have to be formed anew from the filtered out pure sine wave excitation signal in order to eliminate the phase errors caused by the highly selective filter. Thus, some additional electronic circuits are needed, but the complexity of a circuitry is extremely undesirable in implantable medical devices in connection with which the compactness and low current consumption is required.
In addition, the described solution is not suitable for implementing in modern CMOS technology because several electronic blocks operate in near to linear mode.